There has conventionally been known an auxiliary power supply system that is capable of, if needed, pulling out electric power accumulated in a battery or an electric vehicle driven by electric power of a battery. A battery provided to such device is charged mainly by a household or industrial external AC power supply. In this case, an electric leakage might occur at the time of charging, due to a poor connection between the charging line and the device or an insulation defect caused around the battery of the device. It is desired that such electric leakage is detected in an early stage for the reason of an electric shock, breakdown of the device, and detraction of the charging efficiency. Moreover, in a structure where the device is insulated from the ground, an electric charge caused by electric leakage at the time of charging flows to the ground through a human body, causing a discomfort feeling.
Patent Document 1 discloses a charging circuit that has an earth-leakage breaker for detecting a leakage at the time of charging, and a wall socket. This earth-leakage breaker is provided between an external power supply and a charge relay of an electric vehicle has a hall element, an early-leakage relay, and a detecting circuit. The hall element outputs an electric signal when a charging circuit is short-circuited, and the detecting circuit brings the earth-leakage relay from its closed stated into an opened state to break the charging circuit, in response to the voltage signal from the hall element. However, in Patent Document 1, the detecting circuit is not for detecting a minute earth leakage but for detecting the presence/absence of short circuit of the charging circuit.
As a device for detecting a minute earth leakage, there has conventionally been known an earth-leakage monitor that has a residual current transformer on each of two AC lines from an external AC power supply, and detects the difference between the currents flowing through these lines, to detect a current leakage.
Incidentally, as a charge control circuit for charging a storage battery in general, a circuit for once charging an alternating current from an external AC power supply, or an inverter circuit having a high-frequency switching circuit or the like for performing voltage conversion and then charging is adopted. When performing charging by means of these types of charge control circuits, an excess current temporarily flows through a noise suppression capacitor or the like in order to absorb and balance the voltage difference between the external AC power supply and the storage battery, when the external AC power supply and the storage battery are connected to each other or when charging is started. As a result, there is a risk that the earth-leakage monitor malfunctions. In the prior art, measures for this matter are not taken, and therefore the accuracy of earth-leakage detection has a certain limitation in terms of preventing the malfunction. A similar problem can occur in household, commercial or industrial battery charging that is used in the fields other than a field of charging an in-vehicle battery.    Patent Document 1: Japanese Patent Application Publication No. H11-205909